1. Field of the Invention
The present invention relates to an optical scanning apparatus and a color image forming apparatus using the optical scanning apparatus. The present invention is suitable for a color image forming apparatus such as a laser beam printer (LBP), a digital copying machine or a multi-function printer, which employs an electrophotographic process.
2. Description of the Related Art
Conventionally, in an optical scanning apparatus such as a laser beam printer (LBP), a light beam emitted from a light source unit is subjected to optical modulation in accordance with the image signal, and the light beam that has been subjected to the optical modulation is periodically deflected by an optical deflector constituted, for example, of a polygon mirror. Then, the light beams are condensed by an imaging optical system having fθ characteristics in spots on photosensitive members each serving as an image bearing member, to perform optical scanning and record an image.
Further, in a color image forming apparatus in recent years, in order to satisfy the demand for downsizing the apparatus, one deflecting unit, which had heretofore been provided for each of four colors (yellow, magenta, cyan, black) on one-on-one basis, is shared among the four colors. Specifically, a so-called opposing optical scanning apparatus is adopted in which two imaging optical systems are arranged on each side so as to be symmetric with respect to the rotational axis of the optical deflector. Then, a plurality of reflection mirrors are used in each optical path to fold the optical path to guide the light beam to each of the photosensitive members each serving as an image bearing member. Further, as color image forming apparatus in which the optical scanning apparatus are mounted, there have been manufactured ones of a type including a so-called cartridge, in which a photosensitive drum and a developing unit are integrated and exchangeable at once.
Various types of such optical scanning apparatus have been conventionally proposed (see Japanese Patent Application Laid-Open No. 2005-091966).
In the optical scanning apparatus proposed in Japanese Patent Application Laid-Open No. 2005-091966, the optical paths of the imaging optical systems arranged on both sides of the deflector by the reflection mirrors are configured to have arrangements that are different between the imaging optical systems on the right and left of the deflector. This configuration increases the flexibility in arranging the optical paths, so that the optical paths may be folded in sizes of the optical scanning apparatus that are suitable for various color image forming apparatus, to thereby provide an effect that the color image forming apparatus may be downsized.
Meanwhile, in the color image forming apparatus, the frequency at which monochrome images are formed using only the black toner is higher than the frequency at which color images are formed, and hence there is a problem in that only the number of times the black cartridge is exchanged is increased due to the depletion of the black toner or the expiration of the life of the black photosensitive drum.
In order to address the problem, the toner capacity in the black cartridge may be increased, or the diameter of the black photosensitive drum may be increased, to thereby reduce the frequency of exchanging the black cartridge. However, the optical scanning apparatus designed in compliance with the increased size of the cartridge has a longer optical path length. Consequently, when a minimum number of reflection mirrors is to be used to arrange the optical paths, there is a problem in that the optical scanning apparatus is increased in size.
In addition, from the viewpoint of making outer shape of the cartridge of each color uniform, the incident angles of the light beams on the photosensitive drums must be the same, and hence the arrangement of the optical paths in the optical scanning apparatus is severely limited in flexibility. Further, contact surfaces between the photosensitive drum of each color and the conveyer belt must be arranged on line in a sub-scanning section. Therefore, in order to increase only the capacity of the black cartridge, the cartridge is to be increased in size in the parallel arrangement direction or the perpendicular direction of the photosensitive drums while the photosensitive drums remain in the parallel arrangement.
In Japanese Patent Application Laid-Open No. 2005-091966, the black cartridge is placed at the most end position among the cartridges of four colors. In the case where the cartridge capacity is increased in the parallel arrangement direction of the photosensitive drums, the black photosensitive drum is further away from the deflecting unit, to thereby lead to a problem in which the light beam does not reach the photosensitive drum. It is conceivable to move the deflecting unit toward the black photosensitive drum so that the light beam reaches the photosensitive drum. However, this idea is undesirable because decentering the deflecting unit in the optical scanning apparatus results in that the imaging lens of the imaging optical system and the light beam folded by the reflection mirrors interfere more easily. The idea is undesirable also because thermal shift effects are generated asymmetrically on the right and left of the deflecting unit.
In addition, in the case where the black cartridge capacity is increased in the perpendicular direction of the photosensitive drums, the black cartridge intrudes into the optical scanning apparatus, which leads to a problem in which the optical path and the wall surface of the optical scanning apparatus interfere with each other. Otherwise, even in the case where the optical path length is increased in compliance with the increase in the cartridge capacity, unnecessary space is generated between the cartridges other than the black cartridge and the optical scanning apparatus, which leads to an increase in size of the color image forming apparatus.